1. Field of the Invention
This invention relates in general to superconducting quantum interference devices, known as SQUIDs, used for measurement of a magnetic flux. In particular, it relates a new, complex type of superconducting quantum interference device and circuit.
2. Background Art
Superconducting QUantum Interference Devices (SQUIDs) are the most sensitive magnetic field detector known. They involve several quantum phenomena, namely, Josephson tunneling, flux quantization, and quantum interference. A SQUID can be configured to measure a minute change of any physical quantity that can be converted to a flux, such as voltage, current or magnetic field; i.e. it is extremely versatile, being able to measure the magnetic field produced by any of various field sources. Such devices can have energy resolution capabilities approaching the quantum limit. As a result, the SQUID has been used in a wide variety of applications, ranging from medical diagnosis to scientific research, and from non-destructive evaluation to routine measurements of magnetic properties of materials.
There are two kinds of SQUIDs. The first, the dc SQUID, usually consists of two Josephson Junctions connected in parallel on a superconducting loop. The second, the rf SQUID, usually involves a single Josephson Junction on a superconducting loop. In both types, the Josephson Junction is formed by a thin insulating barrier or layer between two pieces or sections of the superconductive loop. The insulating barrier's thickness and cross-section are so very much smaller than the dimensions of the complete circuit loop that electron pairs can tunnel from one side of the Junction to the other without transfer of energy; i.e. when the thickness of the Josephson Junction reaches the order of the coherence length of electron pairs, a superconducting tunnelling current occurs, which is a striking evidence for the existence of long-range order in the superconducting state. This makes it possible to have current flow in the absence of an applied voltage. Particularly in the dc, two Junction loop SQUID, the current produced by interference oscillates with changes in the magnetic field.
Typically, a SQUID produces an output voltage which varies in a periodic manner in response to a small input flux. The extreme sensitivity of such a device derives from the fact that the SQUID can resolve a small fraction of .phi..sub.0, the quantum of magnetic flux, while .phi..sub.0 is itself a very small quantity. Both dc and rf SQUIDs are used, and can be used, as sensors in a wide variety of instruments.
A multi-channel array of SQUIDs was first proposed by Feynman in The Feynman Lecturers on Physics-Quantum Mechanics, Addison Wesley Publishing Co., 1965, Ch. 21, and pursued by various research groups; see J. E. Zimmerman and A. H. Silver, Macroscopic Quantum Interference Effects Through Superconducting Point Contacts, Physical Review, Vol. 141, No. 1, January 1966, and W. H. Chang, Interferometric Measurement of Magnetic Flux, IBM Technical Disclosure Bulletin, Vol. 25, No. 6, November 1982, pp. 2940-2941.
In general, a SQUID can be used to sense a magnetic field directly. However, in many circumstances when this is not a practical arrangement, the SQUID is magnetically coupled to the field by means of a flux transformer. The flux transformer is a closed superconducting circuit which has a primary for sensing the field and a secondary which is magnetically coupled to the SQUID. The principle reason is simply the need to minimize detection of ambient noise. A detection coil responds to the applied field regardless of the distance of its source. With the detection coil of first order or higher, it makes much less demands with regard to screening. The signal thus generated in the SQUID is then fed via electrical leads of normal conducting material to be connected to electronic circuitry for further processing. For some applications, it is of advantage to integrate the external circuit along with the SQUID on a single substrate. There are also situations in which it is possible to configure the SQUID itself to perform some particular function, such as in a magnetometer, gradiometer, susceptometer, or voltmeter. More detailed principles can be found in earlier reviews by Clarke, SQUIDs: Principles, Noise, and Applications, in Superconducting Devices, edited by Steven T. Ruggiero and David A. Rudman, Academic Press, Inc., 1990, pp. 51-99, and Clarke, SQUIDs: Theory and Practice in the New Superconducting Electronics, edited by H. Weinstock and R. W. Ralston, Kluwer Academic Publishers, 1993.
Up to late 1986, it was believed that superconductivity above a temperature of 40 degrees kelvin (k) was not possible, according to the BCS theory. A breakthrough in critical temperature in superconductivity was made in November 1986, by Bednorz and Muller; see J. G. Bendnorz and K. Alex Muller, Z. Phys., B 64, 189, 1986. Now the transition temperatures of new superconductors reach well above one hundred degrees kelvin; see Hasen et al., Phys. Rev. Lett. 60, 1657, 1988, and Parkin et al. Phys. Rev. 60, 2539, 1988. Since the discovery of High-Tc Superconductors, the basic mechanism for high-Tc superconductivity has been far from understood, which may well be somewhat different from the mechanism of low-Tc superconductivity, if not completely different. Although today we have very limited understanding of high-Tc materials, it has generally been accepted that SQUIDs are the most promising application for these materials. With the critical temperature of High-Tc Superconductor (HTSC) now above the boiling point of liquid nitrogen, the HTSC SQUID can operate in liquid nitrogen, which is much less expensive cryogen with a much higher heat capacity as compared to liquid helium. This drastically reduces the operating cost for the device. Such a High-Tc SQUID certainly is more versatile, feasible and economically practical in application.
Unfortunately, realization of this application has been hampered due to a number of unsolved technical challenges.
The first problem is lack of a reliable technique for fabricating the Josephson Junction; this is the essential element of a vast majority of traditional superconducting electronics. A classic Josephson Junction is extremely difficult to fabricate with HTSC. The main reason for this problem is the very small coherence length of the oxide superconductors which is typically on the order of 1 nm. Hence, to fabricate a good Junction it is necessary to have a S-I interface on an atomic scale. The alternative weak link Junction structure, relying on such linear dimensions, is very hard to fabricate even with the most sophisticated lithographic tools available today. Though dc-SQUIDs fabricated with grain boundaries have shown some substantial progress, it is not clear that such a technique can be extended to producing complex circuits.
The second problem concerns the 1/f noise level in HTSC SQUIDs; it is very high compared with that of low temperature SQUIDs. This noise clearly originates in the Josephson elements, and not in the epitaxial HTSC films, for reasons that are not understood.
The third problem is that the characteristic voltage is low, which might be due to the weak tunnelling current. This further lowers the signal-to-noise ratio.
It is difficult to imagine that HTSC SQUIDs will have significant practical applications unless there is a major improvement in their signal-to-noise ratio and sensitivity. In order to achieve a high-performance SQUID with high-Tc superconductors, it is important to enhance the magnitude of the signal gain and the S/N ratio. Equally important is to improve the pattern of oscillation, i.e. the slope of the curve.
Accordingly, an object of the present invention is to provide a SQUID design, particularly a HTSC SQUID design, which will achieve a high current gain, high signal-to-noise ratio and high sensitivity.
These and other objects of the invention will be apparent to those of ordinary skill in this field from the following description of preferred embodiments.